1. Field of the Invention
The present invention relates to an organic light emitting display including a transparent cathode, and more particularly, to an organic light emitting display using a transparent cathode for top emission.
2. Description of the Related Art
Generally, organic light emitting displays include organic light emitting diodes (“OLEDs”) deposited on thin film transistors (“TFTs”) of an electric circuit so that a light emitting layer of a selected OLED emits light in response to a signal controlled by a TFT.
FIG. 1 shows the typical structure of a conventional organic light emitting display. As described above, the conventional organic light emitting display includes an OLED 20 deposited on a TFT layer 10. The OLED 20 includes an anode 21, a light emitting layer 22 and a cathode 23, while the TFT layer 10 includes a substrate 11, a gate electrode 12, a source region 13, a drain region 14 and a via hole 16. Therefore, if a voltage is applied to the gate electrode 12, a current passes through the source region 13 to the drain region 14 while a channel is opened in an organic semiconductor region 15. A current flows to the anode 21 of the OLED 20, the light emitting layer 22 and the cathode 23 through the via hole 16. At this time, in the light emitting layer 22, the action of emitting energy in the form of light occurs as a result of electric coupling, i.e., as a result of the light emitting layer being excited by the recombination of holes and electrons. For example, this light becomes an luminous spot corresponding to one pixel of a display panel.
Reference numeral 30 denotes a passivation layer formed on OLED 20 AND is a protective layer having waterproofing and isolation functions.
In bottom emission devices, light is emitted downward towards the substrate 11, and in top emission devices, light is emitted in the opposite direction. Currently, top emission devices radiating light over a wider area are preferred. That is, in bottom emission devices, the light can be radiated through the gate electrode 12. However, in top emission devices, the light can be emitted through the entire region between lines of a black matrix 24, and thus can provide higher luminance. In order to implement this kind of top emission, the cathode 23 has to be made of a transparent material since the cathode 23 is deposited on the light emitting layer 22. To this end, the cathode 23 is formed of an indium tin oxide (ITO) layer 23b stacked on a silver layer 23a. The silver layer 23a is formed to a thickness of about 5 nm and the transparent ITO layer 23b is formed to a thickness of about 50 nm to about 200 nm on the silver layer 23a. The cathode 23 has the electric conductivity of metal and light transmission of ITO. That is, the cathode 23 fundamentally has an excellent light transmission characteristic and an appropriate conductivity so as not to increase power consumption. Thus, the cathode 23 is formed as a stack of the ITO layer 23b and the silver layer 23a so as to have these two characteristics.
However, according to an experimental result, such a conventional cathode can transmit only 63% of light on average, as indicated in the graph shown in FIG. 2, and thus luminance is insufficient. The light transmittance can be slightly increased by decreasing the thickness of the silver layer 23a. However, this results in a reduction of electric conductivity, and thus power consumption is increased.
Therefore, to solve these problems, a new transparent cathode having an appropriate conductivity and transmittance is desired.